September 10, 2013

Climate Change Extends Mosquito Season, But With Smaller Populations

Climate change's influence on temperature and precipitation is varied, and a new study from the University of Arizona shows that the effect on the spread of West Nile virus might be just as wide-ranging. This suggests that public health efforts to control the virus will need to take a local, rather than global perspective.

Cory Morin and Andrew Comrie, researchers from the University of Arizona, developed a climate-driven mosquito population model to simulate the abundance across the southern United States of one type of mosquito known to carry and spread West Nile virus to humans. The findings, published in Proceedings of the National Academy of Sciences (PNAS), revealed that under the future climate conditions predicted by climate change models, many locations will see a lengthening of the mosquito season but shrinking summer mosquito populations. These changes will be due to hotter and dryer conditions allowing fewer larvae to survive.

The researchers say that these changes vary significantly depending on temperature and precipitation. Decreases in mosquito populations, for example, are expected to be significant in the South, but not further north where there will still be enough rain to maintain summer breeding habitats and extreme temperatures are less common. The study suggests that programs designed to control populations of disease and disease transmission studies must be targeted locally to maximize their effectiveness, the authors argue.

"It used to be an open question whether climate change is going to make disease-carrying mosquitoes more abundant, and the answer is it will depend on the time and the location," said Morin, who did the study as part of his doctoral dissertation in the lab of Andrew Comrie, UA Provost and professor in the UA's School of Geography and Development. Morin is now a postdoctoral researcher in Comrie's team.

"One assumption was that with rising temperatures, mosquitoes would thrive across the board," Morin said. "Our study shows this is unlikely. Rather, the effects of climate change are different depending on the region and because of that, the response of West Nile virus transmitting mosquito populations will be different as well."

"The mosquito species we study is subtropical, and at warmer temperatures the larvae develop faster," Morin explained. "However, there is a limit – if temperatures climb over that limit, mortality increases. Temperature, precipitation or both can limit the populations, depending on local conditions."

Hotter and drier summers in the southwestern US, for example, are expected to delay the onset of mosquito season. The late summer and fall rains, however, are expected to result in a longer season. In contrast, fewer mosquito days will be seen in the south-central US because of less rain during summer and early fall. Except in the Southwest where severe drying inhibits population development during the spring, higher temperatures projected for the shoulder seasons – spring and fall – will likely make for a longer mosquito season across much of the US.

The study focused on one important part in West Nile virus' infectious cycle – mosquitoes of the species Culex quinquefasciatus – however, Morin pointed out that there are other mosquito species that transmit the virus. Another factor not included in the model simulations of the infectious cycle was the fact that the virus also infects birds.

Culex quinquefasciatus is a so-called container breeder that lays its eggs in small volumes of standing water, with larvae that depend heavily on precipitation. Species that prefer larger bodies of water, such as lakes, do not depend so heavily on precipitation.

Statistics from the US Centers for Disease Control and Prevention (CDC) show that 70 to 80 percent of people infected with West Nile virus do not develop symptoms, while the remaining 20 percent will have flu-like symptoms for a week or two. The severest symptoms are limited to less than 1 percent of infected individuals. These symptoms include encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord) and mostly affect the elderly and individuals with compromised immune response.

West Nile virus was first detected in North America in 1999, and since has spread across the continental US and Canada. Humans infected with the virus have been documented in all 48 contiguous states of the US. The region of major epidemics varies from year to year, with the most recent outbreak occurring in Texas in 2012. The CDC reported 1,868 cases in that epidemic.

"'Which locations are likely to experience epidemics in the future?' – those are the kinds of questions studies like ours may help prepare for," said Morin. "We don't model the actual virus, we only look at the vector, but our study informs at least one part of the ecology of the virus. It is unique in projecting the impacts of climate change on a West Nile vector."

The study findings could assist managers and decision makers to better anticipate how mosquito populations will respond to changes in climate and prepare accordingly.

"For example, if projected precipitation and temperature changes for a given area are indicating a longer mosquito season, public health officials can plan to adapt to that possibility through abatement and awareness campaigns."